1. Field of the Invention
The invention relates to generally to parachutes and more particularly to ram air parachutes having an improved canopy design providing reduced opening forces.
2. Discussion of Related Art
Parachutes have evolved over the years into highly sophisticated systems, and often include features that improve the safety, maneuverability, and overall reliability of the parachutes. Initially, parachutes included a round canopy. A skydiver was connected to the canopy via a harness/container to suspension lines disposed around the periphery of the canopy. Such parachutes severely lacked control. The user was driven about by winds with little mechanism for altering direction. Furthermore, such parachutes had a single descent rate based upon the size of the canopy and the weight of the parachutist. They could not generate lift and slowed descent only by providing drag.
In the mid-1960's the parasol canopy was invented. Since then, variations of the parasol canopy have replaced round canopies for most applications, particularly for aeronautics and the sport industry. The parasol canopy, also known as a ram air canopy, is formed of two layers of material—a top skin and a bottom skin. The skins may have different shapes but are commonly rectangular or elliptical. The two layers are separated by vertical ribs to form cells. The top and bottom skins are separated at the lower front of the canopy to form inlets. During descent, air enters the cells of the canopy through the inlets. The vertical ribs are shaped to maintain the canopy in the form of an airfoil when filled with air. Suspension lines are attached along at least some of the ribs to maintain the structure and the orientation of the canopy relative to the pilot. The canopy of the ram air parachute functions as a wing to provide lift and forward motion. Guidelines operated by the user allow deformation of the canopy to control direction and speed. Ram air parachutes have a high degree of maneuverability.
Canopies are flexible and stretchable membrane structures, they distort based upon mechanical and aerodynamic tensions, stresses, airflows and pressure distribution. Although a cell is modeled as having a basically rectangular cross section, when inflated the shape distorts towards round with complex distortions. Typically, in a ram air parachute, suspension lines are not attached to every rib, thus creating loaded ribs (i.e., ribs to which suspension lines are attached) and non-loaded ribs (i.e., ribs which do not have suspension lines attached thereto). Non loaded ribs will float higher than the loaded ribs creating an additional certain amount of span-wise aerodynamic distortion on the top skin of the airfoil. This distortion is aerodynamically undesirable and reduces the efficiency and performance of the canopy.
In order to keep the loaded and non-loaded ribs level and to improve upon the aerodynamics of the canopy, cross-bracing between ribs has been added to some canopy designs. Cross bracing is the use of diagonal ribs in addition to vertical ribs to create more loaded rib-top skin junctions without adding more lines which would increase drag and possible deployment malfunctions. Perfection of the top profile of the airfoil is far more important aerodynamically than the bottom profile. U.S. Pat. No. 4,930,927 illustrates such a design. Cross-braced designs suffer from a number of drawbacks. Cross-bracing results in very complicated construction, high manufacturing costs, and increased packing volume. The standard cross braced design is a ‘tri cell’ construction with a packing volume approximately twenty-five (25) % larger than an equivalent non-cross braced design. Furthermore, the increased rigidness induced by the cross-bracing creates higher opening forces for the pilot. Typically, large cross porting is used on all of the cells to reduce pack volume which does nothing to slow the canopy's inflation on deployment. The opening forces can be so severe that they can jar the jumper's body causing discomfort and even serious injuries. Although designers have implemented “formed” noses, larger sliders, moved bridal attachment points and modified line trims to try to soften the openings of such cross-braced canopies, it has generally yielded only limited improvement.
Prior art canopies use cross porting in their ribs. Upon deployment of the canopy air enters the center cell inlet first and then rapidly inflates the canopy through the crossports and presenting inlets. Slow motion video will show that canopies substantially fully inflate before the slider is pushed ⅓ to ½ of the way down the suspension lines. As the lines are restricted by routing through the slider grommets, the inflating canopy can't maintain a straight leading edge. Instead it violently snakes until the slider is pushed down far enough. This snaking is undesirable causing off heading deployments and in higher performance elliptical planforms frequent line twists or spinning malfunctions.
This method in which a ram air canopy inflates once deployed in freefall has remained basically unchanged for thirty (30) years since the invention of slider reefing. At that time the terminal velocity of a jumper or max speed a jumper would reach before deploying their parachute was typically one hundred twenty (120) mph. In the last ten (10) years new flying disciplines has led to significantly higher terminal velocities. In the early days the only style of body flight in freefall was a belly to earth position that limited terminal velocity to about one hundred twenty (120) mph, now a days ‘freeflyers’ fall with their body positioned vertically typically head down and fall at average speeds of one hundred forty to one hundred eighty (140-180) mph, and ‘speed’ divers fly in vertical positions with arms and legs tucked in streamlined and can reach speeds of over three hundred (300) mph. At speeds over one hundred twenty (120) mph with prior art canopies a jumper runs the serious risk of injury and even death from the increased opening shock. Prior art canopies open taster as your speed at deployment increases. Jumpers who fly at higher speeds endeavor to slow down during freefall before deploying the parachute by transitioning to a belly to earth position and waiting, but they often do not slow sufficiently to achieve acceptable opening forces. Additionally there is always the common possibility of a premature deployment where your canopy is deployed unintentionally early in the dive. Therefore, a need exists for a safer parachute providing reduced opening force at the higher decent rates being practiced.